That’s not true though. Helium doesn’t just rise through the atmosphere and gets lost to space. A helium balloon rises because it’s less dense than air, so air pushes up on it. It rises until the atmosphere is thin enough and stops there. When helium is not in a balloon, it doesn’t rise because it mixes with air and air doesn’t push on it. The atoms are still smaller and move faster than other gases. Some will go up and eventually gain enough speed to hit escape velocity. According to Maxwell-Boltzmann speed distribution of noble gases, only a small fraction of helium should be escaping the earth atmosphere due to that. The actual amount escaping is larger than predicted, but the exact mechanism isn’t fully agreed upon. Solar winds are thought to be responsible, but that’s just one theory. But the important thing is that helium doesn’t just rise when mixed with oxygen or nitrogen a.k.a “air”
A quick search seems to show that helium is being lost to space. Wikipedia’s article claims the loss of helium to be at a rate of about 50 grams per second.
I was talking about the mechanism it’s lost to space by not denying it. It doesn’t simply rise until it escapes like a helium balloon. Solar wind and helium kinetic energy play a bigger role there.
